This invention relates to a process for recovering tungsten as an ammoniacal tungstate compound by a process which involves forming sodium tungstate from a starting tungsten bearing material, crystallizing and redissolving the sodium tungstate, extracting tungsten by an organic extracting agent, and subsequently stripping the tungsten from the organic with ammonia to form an ammoniacal tungstate solution. This solution is crystallized to form an ammoniacal tungstate compound. The process makes use of a procedure in which the mother liquor resulting from the crystallization of the ammoniacal tungstate compound is used in redissolving the sodium tungstate. Reuse of the mother liquor in this manner affords environmental and economic advantages.
Tungsten is manufactured either from ores or scrap sources. If scrap material is the starting material it is converted to an oxide before the digestion step. Digestions are carried out at either atmospheric pressure or higher pressures depending on the type of material, with an excess of sodium hydroxide. During the digestion the tungsten is converted into an impure sodium tungstate. To remove excess sodium hydroxide, the digestion solution is evaporated to form sodium tungstate crystals. This is done preferably in continuous crystallizer which separates the sodium tungstate crystals from the sodium hydroxide. The sodium hydroxide is recycled back to the starting (digestion) process. At this point the impure sodium tungstate crystals are redissolved in water and treated to remove the impurities such as molybdenum, arsenic, silica, and phosphorus. The solution is further processed by liquid-liquid extraction in which the tungsten is extracted from the sodium tungstate solution by an organic extracting agent. The tungsten is then stripped from the organic with ammonia to form an ammoniacal tungstate solution. An ammoniacal tungstate compound is then crystallized from this solution. The resulting mother liquor contains tungsten and a fairly high concentration of impurities. Prior to the present invention this mother liquor was recycled to the sodium tungstate recrystallization step at which point it was added to the digestion solution prior to the crystallization of the sodium tungstate. Because the mother liquor contains a large amount of ammonia which is released during the crystallization by the sodium hydroxide, it causes major venting problems and lower yield of sodium tungstate crystals.
It would be an advantage both environmentally and economically to eliminate the problems caused by the release of ammonia and at the same time to recycle the mother liquor.